1. Technical Field
The present invention relates to an electrostatic image developer, a developer cartridge, a process cartridge, an image forming apparatus and an image forming method.
2. Related Art
A method for visualizing an image information through an electrostatic latent image (electrostatic image), such as electrophotography, is currently used in various fields. In the electrophotography, an electrostatic latent image formed on a photoreceptor by charging and exposure steps is developed with an electrostatic image developer (hereinafter sometimes referred to as a “developer” for simplicity) containing an electrostatic image developing toner (hereinafter sometimes referred to as a “toner” for simplicity), and is visualized through a transfer step and a fixing step. A developer used in development includes a two-component developer containing a toner and a carrier for an electrostatic image developer (hereinafter sometimes referred to as a “carrier” for simplicity), and a one-component developer using a toner alone such as a magnetic toner. In the two-component developer, a carrier shares functions of stirring, transportation, charging and the like of a developer, and thus is functionally separated. As a result, the two-component developer has the characteristics of good controllability, and is widely used at present.
In such a two-component developer, investigations of reducing a particle diameter are made in a toner to obtain a full color image having high image quality and high definition. Furthermore, investigations are made in a carrier to control the carrier to a range showing semiconductivity from the standpoint of electric resistance.
Reduction of a particle size of a toner is initially advantageous to high image quality. However, when external additives on the surface of a toner having a small diameter are buried in the toner, transfer efficiency is remarkably decreased, easily causing deterioration of image quality. Furthermore, investigations of fixing a toner at low temperature are made to achieve powder saving. However, a toner which is easily fixed at low temperature tends to decrease its hardness, and external additives are susceptible to be buried by stirring in a developing device. For the stabilization of developability, transferability and cleanability of a toner having a small diameter, a toner which fixes at low temperature and a toner which fixes under pressure, a carrier is required to have low stress to a toner.
In general, a carrier is preferably appropriately amorphous to stably obtain developer transportability by a developer supporter. When the carrier is too spherical, carrier particles themselves induce slip, resulting in deterioration of transportability. On the other hand, when the carrier is too amorphous, such a shape adversely affects chargeability imparting function and charging sustainability.
Further, in general a carrier preferably has low specific gravity to reduce stress imparted to a toner. In this case, various properties described above required in a carrier must be maintained. To achieve this, many magnetic substance dispersion type carriers are recently proposed. The magnetic substance dispersion type carrier permits to decrease specific gravity of carrier particles while maintaining magnetic properties by dispersing a magnetic powder having strong magnetic properties in a resin having small specific gravity. However, the carrier mentioned above shows certain effects in transportability of a toner and reduction in stress to a toner, but the effects are low.